In recent years, further improvements have been expected in the performance of rechargeable batteries used in fuel cell electric vehicle and a hybrid vehicle. Therefore, in order to prevent overcharge and overdischarge of rechargeable batteries and improve the cycle life thereof, techniques for accurately calculating the state of charge (hereinafter, referred to as SOC) of the battery are required.
Hereinafter, SOC which is calculated on the basis of current value is referred to as SOCI, while SOC which is calculated on the basis of voltage value is referred to as SOCV. A conventional battery state-of-charge calculation device integrates a value of electric current charged and discharged so as to calculate SOCI, which is given as the SOC of the battery. However, measurement errors of the value of electric current are gradually accumulated in SOCI because the value of electric current is integrated. Therefore, a problem of SOCI deviating from actual SOC is arisen.
Therefore, in a conventional battery state-of-charge calculation device, first, SOCI is used as SOC and thereafter, SOCV which is calculated on a timely basis is used as SOC in place of SOCI to correct SOC. For example, in the battery state-of-charge calculation device disclosed in Patent Document 1, upon detecting switchover from charge to discharge in a battery and vice versa, change in voltage values resulting from internal resistance of the battery (hereinafter, referred to as estimated impedance voltage value) is calculated from an actually measured current value and an actually measured voltage value, and an estimated no-load voltage value is calculated from the estimated impedance voltage value and the actually measured voltage value. Upon detecting switchover from charge to discharge in a battery and vice versa, the device of Patent Document 1 choices one of SOCV based on the thus calculated estimated no-load voltage value and SOCI based on an integrated current value, and the device uses the chosen one as SOC.
Further, in the battery state-of-charge calculation device disclosed in Patent Document 2, an estimated impedance voltage based on an actually measured temperature and an actually measured current value of a battery is subtracted from an actually measured voltage value, thereby resulting in an estimated no-load voltage value. The device of Patent Document 2 calculates SOC using SOCV on the basis of the thus calculated estimated no-load voltage value, along with SOCI on the basis of an integrated current value.